Cellular communication systems are known. Such systems are, typically, comprised of a number of remote base sites, each having a service coverage area, and a number of cellular telephones (communication units). Remote base sites are typically dispersed throughout a geographic area providing local communication services to communication units passing through the area from the nearest remote base site. Within the geographic area, service coverage areas of adjacent remote sites are often arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a communication unit receiving service from one remote site may be handed off to an adjacent remote site with no interruption in service. The Groupe Special Mobile (GSM) Pan-European cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI) and incorporated herein by reference, is an example of just such a system using such a format.
Such systems, typically, provide communication access to communication units within service coverage areas (cells) on an assigned frequency spectrum. The assigned frequency spectrum is divided into a number of operating frequencies (f.sub.1, f.sub.2, . . . f.sub.7n+7).
To maximize the capacity of a cellular system within a given geographic area, operating frequencies must be reused among the cells in such a manner that mutual interference does not exceed a maximum threshold level. Mutual interference, within such systems, is maintained below the maximum threshold level through the use of a reuse pattern.
Under the reuse pattern a list of available, idle channels is maintained at each remote base site serving a cell. The list of available frequencies is, in turn, further divided into a list of useable channels based upon interference measurements.
The list of useable channels available at remote base sites, for handoffs or for allocation to communication units requesting access, is determined from interference measurements performed by the base sites and comparison with a cell threshold (or thresholds for defining degrees of channel quality). Channels having measurements below the highest threshold may be deemed useable while those above the threshold may be deemed not useable.
The threshold method of determining channel availability by comparison of interference measurements with thresholds works well in most cases. Communication units requesting access from locations near the base site typically experience good carrier to interference (C/I) ratios and clear audio channels. Problems arise, on the other hand, where a requesting communication unit is located near the periphery of a service coverage area or is in a poor service area. In such a case the communication unit may be operating at, or near, full output power yet still providing a signal that appears weak to the base site.
In the case where a communication unit is near the periphery of the service coverage area (because of handoff or otherwise), the allocation of a communication channel, under the threshold method, may result in a poor quality of service. In specific, if high quality channels (with low measured interference) are used indiscriminantly for calls with strong signals, then calls with weak signals may be left with only poor quality channels which may produce poor audio quality, service interruptions, and dropped calls.
Because of the importance of mobile telephone communications a need exists for a better method of selecting channels for assignment during requests for access or requests involving handoff. Such a method should take into account the vagaries of signal quality associated with geographic location.